applyInputHandlers: Apply input handlers to raw input values
Description
The purpose of this function is to make it possible for external packages to
test Shiny inputs. It takes a named list of raw input values, applies input
handlers to those values, and then returns a named list of the processed
values.
The raw input values should be in a named list. Some values may have names
like "x:shiny.date". This function would apply the "shiny.date"
input handler to the value, and then rename the result to "x", in the
output.